1 PYRAMIDS - CORTICOSPINAL FIBERS PYRAMIDS - CORTICOSPINAL FIBERS ... drawing that will account for...

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1 PYRAMIDS - CORTICOSPINAL FIBERS

Pyramids/Corticospinal Tract

The pyramids are two elongated swellings on the ventral aspect of the medulla. Eachpyramid contains approximately 1,000,000 CORTICOSPINAL AXONS. As the name suggests,these axons arise from the cerebral cortex and descend to terminate within the spinal cord. Thecortical cells that give rise to corticospinal axons are called Betz cells. As corticospinal axonsdescend from the cortex, they course through the internal capsule, the cerebral peduncle of themidbrain, and the ventral pons (you will learn about these structures later in the course so don’tworry about them now) and onto the ventral surface of the medulla as the pyramids (see below).

When corticospinal axonsreach the medulla they lie withinthe pyramids. The pyramids arejust big fiber bundles that lie onthe ventral surface of the caudalmedulla. The fibers in thepyramids are corticospinal. It isimportant to REMEMBER:THERE HAS BEEN NOCROSSING YET! in thissystem. The cell bodies ofcorticospinal axons within thepyramids lie within theIPSILATERAL cerebral cortex.

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At the most caudal pole of the pyramids the corticospinal axons cross over themidline and now continue their descent on the contralateral (to the cell of origin) side. Thiscrossover point is called the PYRAMIDAL DECUSSATION. The crossing fibers enter the lateralfuniculus of the spinal cord where they are called the LATERAL CORTICOSPINAL TRACT(“corticospinal” is not good enough, you have to call them lateral corticospinal; LCST - rememberthis one??). LCST axons exit the tract to terminate upon neurons in the spinal cord gray matteralong its entire length. Some of the spinal cord neurons that receive direct input from lateralcorticospinal fibers send axons into the ventral root to innervate striated muscles that move the armsand legs. In such instances only two neurons and one synapse are involved (monosynaptic). Lateralcorticospinal tract axons also end upon spinal cord neurons that do not directly innervate muscle, butinstead synapse upon lower motor neurons. Such cells are called interneurons. A pathwayinvolving a lateral corticospinal axon, an interneuron and a lower motor neuron is disynaptic.

Let’s review! The cells of origin of the corticospinal tract lie in the ipsilateral cerebralcortex. In contrast, the cells of origin of the lateral corticospinal tract lie in the contralateralcerebral cortex. REMEMBER—the lateral corticospinal tract is the part of the corticospinal systemin the spinal cord. It is important to remember that damage to the corticospinal fibers rostral to thepyramidal decussation results in contralateral motor deficits, while lesions in the spinal cord (i.e.,caudal to the decussation) result in ipsilateral deficits. Clinicians refer to the motor weaknessresulting from a lesion of the corticospinal tract as hemiplegia (plegia =stroke or paralysis) orhemiparesis (weakness). For instance, they would say that a lesion of the LEFT corticospinal tractin the internal capsule (rostral to the pyramidal decussation) results in a RIGHT hemiplegia(involves the right arm, trunk and leg) while a lesion of the LEFT lateral corticospinal tract at C1 (inthe spinal cord and therefore caudal to the decussation) results in LEFT hemiplegia. Since themotor neurons that directly innervated the muscle are OK, there is no muscle atrophy. This isimportant.

A unilateral lesion of the lateral corticospinal tract results in motor deficits ipsilateral tothe lesion. Interrupting the tract does NOT result in muscle atrophy, since the neurons innervatingmuscle are NOT dead. For example, if the LEFT lateral corticospinal tract is interrupted at Cl, thenthe LEFT arm, trunk and leg are affected. This is because the lateral corticospinal tract influencesthe musculature on the same (ipsilateral) side of the body. If the lesion involves the LEFT lateralcorticospinal tract at T3, then only the LEFT trunk and leg are affected, since the spinal cord motorneurons that innervate the muscles of the arms still receive corticospinal input.

As mentioned above, the term hemiplegia is commonly used by clinicians when discussingthe effects of lesions of the corticospinal tract anywhere from the cortex to the medulla, and of thelateral corticospinal tract anywhere in the spinal cord. Cerebrovascular accidents (“strokes”)commonly damage the corticospinal tract in the motor cortex or the posterior limb of the internalcapsule (a compact bundle of axons through which almost all neural traffic to and from the cortexpasses). The term “stroke” is poorly defined and has different meanings to different users. Itgenerally implies the abrupt onset of neurological deficits as the result of cerebrovascular disease.As such, it includes the manifestations of cerebral hemorrhage (Gr. blood bursting forth; may bearterial, venus or capillary) cerebral infarction (L. to stuff into; area of tissue undergoes necrosis[death] following cessation of blood supply) and intracranial and extracranial thrombosis (Gr. clotcondition; when a thrombus is detached from its original site and found in another site it is called athrombotic embolis [Gr. embole=throwing in]).

As you have already learned, immediately following a lesion involving corticospinal orlateral corticospinal fibers, there is a period of flaccid paralysis (spinal shock). After a period ofdays to weeks, muscle tone (spasticity) and muscle reflexes return and increase. There will also be aBabinski sign, (extension of the big toe and fanning of the others in response to firmly stroking thesole of the foot; “the plantar reflexes are extensor”). There will also be clonus.


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You need to keep in mind that lesions of the corticospinal tract do not destroy motor neuronsthat DIRECTLY innervate muscle. THUS, THERE IS NO DEATH OR ATROPHY OFMUSCLE. Corticospinal tract neurons are referred to as UPPER MOTOR NEURONS since theydo not innervate muscle directly. In contrast, neurons in the ventral horn that DIRECTLY innervatemuscle are called LOWER MOTOR NEURONS (LMN). When these neurons die there isATROPHY OF THE MUSCLE.

In my humble opinion, the corticospinal tract is the most important pathway in clinicalneurology. I will ask you about this tract every time I see you, even after you graduate! In otherwords, if you forget any neuroanatomy, DO NOT forget the following points:

1. the pathway is crossed.

2. lesions anywhere rostral to the decussation (cortex, internal capsule, cerebral peduncle,basilar pons, rostral two-thirds of pyramid) result in contralateral deficits.

3. lesions involving this system in the spinal cord (i.e., the lateral corticospinal tract) resultin ipsilateral deficits below the level of the lesion.

4. corticospinal neurons are upper motor neurons; and therefore their death does NOT resultin muscle atrophy.


PROBLEM SOLVING MATCHING

Match the best choice in the right hand column with the pathway or cell group in the left handcolumn

____1. left pyramid A. lesion results in a left Babinski

B. lesion results in left hemiplegia

C. cells of origin of axons lie in right motor cortex

D. axons arise from cells in the left motor cortex

E. axons terminate on the left side of the spinal cord

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Pyramids/Corticospinal TractProblem Solving

PROBLEM SOLVING

RIGHT LEFTShade in the location of a single, continuous, unilateral (limited to one side) lesion in the above

drawing that will account for the following neurological problems:

right hemiplegia, right Babinski

Brain stem 156Pyramids/Corticospinal TractProblem Solving

PROBLEM SOLVING ANSWER

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Hopefully you remember from the spinal cord module that the cells of origin of pain andtemperature conveying axons in the spinal cord lie in the dorsal horn. Axons arising from thesedorsal horn cells cross and ascend in the anterolateral portion of the white matter of the spinal cord(hence the name Anterolateral System; ALS). Thus the cells of origin of the ALS (or spinotha-lamic tract) lie in the contralateral dorsal horn. Axons in the ALS are destined for the ventralposterolateral (VPL) nucleus of the thalamus. Since the thalamus lies ROSTRAL to the midbrain,which is the most rostral part of the brain stem, the ALS is present in each of the 10 brain stem slidesthat you need to learn.

Once the pain and temperature information traveling in the ALS reaches the VPL nucleusof the thalamus (the thalamus is the GREAT GATEWAY TO THE CORTEX), it is relayed by athalamic neuron to the somatosensory cortex (postcentral gyrus; areas 3, 1 and 2). Don’t forget:pain and temperature information reaches cells in the dorsal horn via the central processes ofdorsal root ganglion cells (delta and C fibers; neuron #1). Dorsal horn cells (neuron #2) project tothe contralateral VPL via the ALS. Finally, cells in VPL (neuron #3) project to areas 3, 1 and 2(somatosensory cortex) for perception of the pain and temperature.

Interruption of the ALS anywhere in the brain stem results in loss of pain and temperaturefrom the contralateral side of the body (arms, trunk and legs), but NOT the head. We will dealwith the head later!

The location of the ALS is difficult to see in fiber-stained sections, since most of these axonsare either lightly myelinated or non-myelinated (i.e., slowly conducting). You must focus hard onremembering the location of the anterolateral fibers in each of the ten brain stem levels. I willinclude this system in many of the problem-solving questions in order to help you remember itslocation. YOU SHOULD NEVER FORGET THE ALS/SPINOTHALAMIC TRACT!

2 ANTEROLATERAL SYSTEM (ALS)

Anterolateral System

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AN INTERESTING CLINICALOBSERVATION

Clinical case reports often comment thatlesions involving areas of the brain stemadjacent to the anterolateral system result

in an IPSILATERAL HORNER’SSYNDROME. The explanation for thisfinding is that descending fibers from theHYPOTHALAMUS (a major autonomiccenter) travel close to the ALS in the brain

stem (you will not see these descendingfibers in the ten brain stem levels). Their

interruption means that thePREGANGLIONIC SYMPATHETIC

neurons in the lateral cell column (T1-L2)of the spinal cord have lost an important

“drive.” The most obvious clinical findingwould be a CONSTRICTED PUPIL

(MIOSIS) in the ipsilateral eye since theparasympathetic input is now in control.

There would also be slight drooping(PTOSIS) of the ipsilateral upper eyelid

due to the absence of sympathetic “drive”to the superior tarsal (smooth) muscle.

There is also lack of sweating (anhidrosis)and vasodilatation (flushed face).

Remember, sympathetics innervate sweatglands and constrict blood vessels of theface. All problems are IPSI to the brain

stem lesion. For our problem solvingexcercises A HORNER’S WILL BE PRESENTANY TIME THE LESION INVOLVES THE

ALS IN THE BRAIN STEM OR THE AREA OFTHE MEDIAL LCST IN THE SPINAL CORDABOVE T2! REMEMBER, THE PROBLEMS

RELATED TO HORNER’S WILL BE IPSI.WHILE THOSE RELATED TO THE ALS

WILL BE CONTRA.


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Match the best choice in the right hand column with the pathway or cell group in the left hand column.

____1. left pyramid A. cells of origin lie in the left dorsal root ganglia

____2. right anterolateral system (ALS) B. axons terminate in the left motor cortex and associated descending pathway

C. lesion results in a loss of pain and temp from the right side of the body

D. lesion results in a left Babinski

E. cells of origin lie in the left dorsal horn

F. cells of origin lie in the right motor cortex

G. lesion results in right hemiplegia

H. axons terminate on the left side of the spinal cord

I. lesion results in a dilated pupil in the right eye

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Anterolateral SystemProblem Solving

PROBLEM SOLVING

RIGHT LEFTShade in the location of a single, continuous, unilateral lesion in the above drawing that will account

for the following neurological problems:

right hemiplegia, right Babinski, loss of pain and temperature from the right arm and leg, constricted pupilin the left eye

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Anterolateral SystemProblem Solving


RIGHT LEFT

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The trigeminal nerve is the largest cranial nerve. It contains both sensory and motorfibers. The motor fibers innervate the muscles of mastication and arise from motor neurons in thepons. The general sensory fibers convey information regarding pain, temperature, touch, andconscious and unconscious proprioception. At this time I want to talk about those general sensoryfibers that are conveying PAIN and TEMPERATURE because they are associated with a nucleusand tract in the caudal medulla (where we are now!). We will talk about the other general sensorynuclei and fibers (two point discrimination, conscious proprioception and vibratory sense), as well asthe motor fibers associated with the trigeminal, later when we look at sections through the rostralpons.

The cell bodies of the pain and temperature fibers associated with the trigeminal nerve lie inthe trigeminal ganglion (located on the cerebral surface of the sphenoid bone in the middle cranialfossa). Just like cells within the dorsal root ganglia of the spinal cord, cells in the trigeminalganglion possess peripheral and central processes. The peripheral processes of trigeminal ganglionneurons distribute to pain and temperature receptors on the face, forehead, mucous membranes ofthe nose, anterior two-thirds of the tongue, hard and soft palates, nasal cavities, oral cavity,teeth, and portions of cranial dura. The central processes enter the brain at the level of the pons(this is where all trigeminal sensory fibers enter the brain stem and where trigeminal motor fibersleave the brain stem). These central processes of trigeminal ganglion neurons conveying pain andtemperature descend in the brain stem and comprise the SPINAL TRACT V. Fibers of spinal tractV terminate upon an adjacent cell group called the SPINAL NUCLEUS V, which forms a long cellcolumn medial to spinal tract V (spinal tract and nucleus V form a slight elevation on the lateralsurface of the caudal medulla called the tuberculum [swelling] cinereum [ashen or gray]). We areparticularly concerned with the caudal-most portion of spinal nucleus V, because ALL of the PAINand TEMPERATURE fibers from the face terminate in this caudal region of the nucleus (otherportions of spinal nucleus V will not be discussed). Cells within spinal nucleus V possess axons thatcurve medially to CROSS the midline and course rostrally close to (but not as a part of) the mediallemniscus. These crossed fibers retain their closeassociation with the medial lemniscus as theyascend in the brain stem and are called thetrigeminothalamic tract (TTT). Neither thecrossing or ascending TTT fibers can be identifiedin our fiber-stained levels, since they are onlylightly myelinated. Fibers in the TTT reach theGREAT GATEWAY TO THE CORTEX, thethalamus, where they terminate in the ventralposteromedial (VPM) nucleus. Neurons in theVPM then project to somatosensory cortex (areas3, 1, 2; postcentral gyrus).

3 SPINAL NUCLEUS AND TRACT OFTHE TRIGEMINAL (C.N. V)

Spinal Nucleus and Tract V

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I am sorry to say that the spinal tract and spinal nucleus V are not exclusively associated withC.N. V. It is also associated with C.N.s VII (facial), IX (glossopharyngeal) and X (vagus). Whilethis makes the story a little confusing, it also makes a lot of sense! The pain and temperature fibersassociated with C.N. VII innervate the skin of the external ear, the wall of the external auditorymeatus and the outer surface of the tympanic membrane (if it’s OK with you, I’ll lump these togetheras “EAR”). These fibers are the peripheral processes of cells that lie in the GENICULATEganglion (located in the facial canal). The central processes of these neurons enter the brain withC.N. VII (at pontine levels, caudal to the trigeminal), travel in spinal tract V and end in spinalnucleus V. The pain and temperature information is then conveyed rostrally in the TTT(trigeminothalamic tract) to reach the VPM, from which it is relayed to somatosensory cortex (areas3,1 and 2). Thus the pain and temperature fibers of C.N. VII don’t have their “own” central cellgroup, but instead use that of the trigeminal.

Like C.N. VII, C.N. IX is involved in the pain and temperature innervation of the “EAR”. Inaddition, C.N. IX conveys pain and temperature from the posterior one-third of the tongue, theauditory tube and the upper part of the pharynx (touching of the pharyngeal wall results in a gagreflex which we will cover in Point 9: Nucleus Ambiguus). The cell bodies of these pain andtemperature axons associated with C.N. IX lie in the relatively small SUPERIOR GANGLION IX(located just outside of the jugular foramen; you will hear about the larger inferior ganglion later).The central processes of cells in the superior ganglion enter the brain with C.N. IX (at the lateralmedulla), then enter our friend spinal tract V and synapse in the caudal spinal nucleus V. You knowthe route from here.

FINALLY (whew!) C.N. X innervatesthe “EAR” (along with C.N.s VII and IX). Inaddition, pain and temperature from the lowerpharynx, the larynx and the upper esophagusare conveyed by C.N. X. The cell bodies ofthese pain and temperature axons lie in theSUPERIOR GANGLION X (located justoutside the jugular foramen). The centralprocesses of cells in the superior ganglion Xpass into the brain at the lateral medulla, enterour old friend spinal tract V and synapse in thecaudal spinal nucleus V. I know you can take itfrom here up to cortex via the thalamus!


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KEY THOUGHT: only ONE ganglion to worry about with C.N. VII (GENICULATE).SUPERIOR ganglia of BOTH IX and X=somatic afferents (inferior ganglia IX and X will be

discussed later, but if you are interested, they are related to visceral afferents; ahhh, those donuts onFriday during Gross tasted great!)

As mentioned above, pain fibers end only in the caudal spinal nucleus V. You should alsobe aware of the fact that not much is known about the other parts of the spinal trigeminal nucleus(nucleus oralis and interpolaris). The important fact is that the great majority of the pain andtemperature fibers pass to the most caudal portion of spinal nucleus V. This means that lesionsanywhere along the rostrocaudal extent of the spinal tract V, as well as in the caudal spinal itself,will result in deficits in pain and temperature from the ipsilateral face etc. Finally, you haveprobably heard of tic douloureux or trigeminal neuralgia, which is a disorder characterized bybrief attacks of severe pain within the distribution of one or more divisions of the trigeminal nerve.Sometimes, section of the spinal nucleus and tract V in the caudal medulla relieves these symptoms.

For our PROBLEM SOLVING, you must know that a lesion of spinal tract V along itsentire course (the pain information never gets to the caudal spinal nucleus) results inIPSILATERAL deficits in pain and temperature from the face etc. Interruption of thetrigeminothalamic tract, which is comprised of axons that have crossed the midline, results indeficits in pain and temperature on the contralateral side of the face etc.

REMEMBER:

1. spinal tract and nucleus V are present in the pons and medulla

2. the cells of origin of spinal tract V lie in the trigeminal ganglion

3. axons in spinal tract V terminate in the spinal nucleus V

4. axons of cells within spinal nucleus V project to the contralateral VPM

5. spinal tract and nucleus V = pain and temp. from face etc. (ear, tongue, palates, pharynx,larynx, upper esophagus)


AN INTERESTING CLINICAL OBSERVATION

Reports in the clinical literature note that vascular lesions that interrupt the blood supply tothe spinal nucleus and tract V in the medulla (for example a thrombosis of the posterior inferiorcerebellar artery) sometimes are immediately followed by sharp stabbing pain in and around the eyeand on the ipsilateral face (hyperalgesia; algesia; Gr., = sense of pain). A possible explanation forthis paradox is that the pain fibers are highly irritated before they die (spontaneous pain alsosometimes occurs on the contralateral side of the body immediately following a lesion of theanterolateral system). Please remember that lesions of spinal tract and nucleus V can result inPAIN in the face (in addition to loss of pain and temperature later). What a paradox!

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Spinal Nucleus and Tract VProblem Solving

PROBLEM SOLVING

RIGHT LEFTShade in the location of a single, continuous, unilateral lesion in the above drawing that will account for

the following neurological problems:

left hemiplegia, left Babinski, loss of pain and temperature from the right arm and leg, sharp pain in theleft eye followed later by loss of pain and temperature from the left side of the face, constricted pupil in

the left eye


Match the best choice in the right hand column with the pathway or cell group in the left hand column

____1. right pyramid A. axons terminate on right side of the spinal cordB lesion results in a left Babinski

____2. right anterolateral system (ALS) C. lesion results in the loss of pain and temp from and associated descending pathways the right side of the body

D. lesion results in the loss of pain and temp from____3. right caudal spinal nucleus V the right side of the face

E. cells of origin lie in the right dorsal hornF. cells of origin lie in the left trigeminal ganglionG. lesion results in right hemiplegiaH. lesion results in a constricted pupil in the right eye I. axons arise from cells in the left motor cortex J. cells project to the right VPM

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Spinal Nucleus and Tract VProblem Solving


1 PYRAMIDS - CORTICOSPINAL FIBERS PYRAMIDS - CORTICOSPINAL FIBERS ... drawing that will account for...

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